The invention relates generally to air conditioning and refrigeration systems, and more specifically to a cryogenic temperature control apparatus and a method of operating a cryogenic temperature control apparatus.
Conventional cryogenic temperature control systems typically store a compressed cryogen such as carbon dioxide, liquid nitrogen, etc. in a pressurized storage tank. The cryogen is directed along a conduit from the storage tank to an evaporator coil that extends through a heat exchanger. Relatively warm air is passed across the evaporator coil and is cooled by the evaporator coil. The cooled air is returned to cargo compartment to pull down the temperature of the cargo compartment to a predetermined set point temperature. The warm air heats and vaporizes the cryogen in the evaporator coil. After the heat transfer has occurred, the vaporized cryogen is typically exhausted to the atmosphere.
Conventional cryogenic temperature control systems typically include a series of sensors which record temperature and pressure values in various locations throughout the system. The sensors generally supply the temperature and pressure data to a controller, which uses an elaborate fuzzy logic scheme to control the operating parameters of the system based upon the data provided by the sensors. In order to achieve and maintain the set point temperature, the controller periodically determines the rate of change of the temperature of the discharge air as well as the acceleration or deceleration of this rate of change. Based upon these and other calculations, the controller increments the flow of cryogen from the storage tank to the evaporator coil by activating and deactivating an electronically controlled valve. Generally, the fuzzy logic schemes are relatively complicated to program and to operate.
The controllers used to operate conventional cryogenic temperature control apparatuses are generally relatively complex. These systems generally require substantial computing power and programming skill to properly implement and operate. Additionally, the system complexity generally limits the flexibility of conventional cryogenic temperature control apparatuses. Also, they generally consume relatively large quantities of cryogen. This is particularly problematic on vehicle mounted cryogenic temperature control apparatuses. Cryogenic temperature control systems are currently used in mobile applications to control the temperature in a cargo compartment and are typically mounted on straight trucks, the trailer of a tractor-trailer combination, a refrigerated shipping container, a refrigerated railcar, and the like. For obvious reasons, it is generally desirable to reduce the weight and size of the cryogenic temperature control system. Often conventional storage tanks can weigh 1400 pounds or more when filled. It is therefore generally desirable to minimize the amount of cryogen that is carried in the storage tank and to reduce the rate at which the cryogen is consumed while ensuring that the air-conditioned space temperature is maintained at or near the set point. Additionally, cryogen may not always be readily available for refilling the storage tank so it is important, particularly during long hauls, to regulate the consumption of cryogen.
According to the present invention, a method of temperature control in a cryogenic temperature control apparatus comprising providing a heat exchanger in thermal communication with an air-conditioned space is provided. The heat exchanger includes an air inlet and an evaporator coil having an outlet. A first temperature sensor is operatively coupled to a controller, measures the temperature in the outlet, and sends the temperature in the outlet to the controller. A second temperature sensor is operatively coupled to the controller, measures the temperature in the air inlet, and sends the temperature in the air inlet to the controller. The invention further comprises providing a first plurality of temperature control values and a second plurality of temperature control values. The flow of cryogen from a storage tank to the evaporator coil is altered each time the temperature in the outlet passes the first plurality of temperature control values each time the temperature in air inlet passes the second plurality of temperature control values.
In preferred embodiments, the method of temperature control includes providing a first cooling mode corresponding to a first flow rate of cryogen from the storage tank to the evaporator coil, providing a second cooling mode corresponding to a second flow rate of cryogen from the storage tank to the evaporator coil, providing a third cooling mode corresponding to a third flow rate of cryogen from the storage tank to the evaporator coil, and providing a fourth cooling mode corresponding to a fourth flow rate of cryogen from the storage tank to the evaporator coil. Altering the flow of cryogen from the storage tank to the evaporator coil when the temperature in the outlet and the temperature in the air inlet are beyond the plurality of temperature control values includes switching between the first cooling mode, the second cooling mode, the third cooling mode, and the fourth cooling mode.
In preferred embodiments, a system for incorporating the method includes a first valve and a second positioned between the storage tank and the evaporator coil for altering the flow of cryogen from the storage tank to the evaporator coil. The first valve has a first position and a second position and the second valve has a third position and a fourth position. The first valve is moved into the first position and the second valve is moved into the third position to provide a first mass flow rate of cryogen from the storage tank to the evaporator coil. The first valve is moved into the first position and the second valve is moved into the fourth position to provide a second mass flow rate of cryogen from the storage tank to the evaporator coil. The first valve is moved into the second position and the second valve is moved into the third position to provide a third mass flow rate of cryogen from the storage tank to the evaporator coil. The first valve is moved into the second position and the second valve is moved into the fourth position to provide a fourth mass flow rate of cryogen from the storage tank to the evaporator coil.
The heat exchanger includes a heating element. The flow of cryogen from the storage tank to the evaporator coil is discontinued each time the temperature in the outlet passes at least one of a third plurality of temperature control values and each time the temperature in the air inlet passes at least one of a fourth plurality of temperature control values. Air in the heat exchanger is heated with the heating element each time the temperature in the outlet passes at least one of the third plurality of temperature control values and each time the temperature in the air inlet passes at least one of the fourth plurality of temperature control values.
Other features and advantages of the invention will become apparent to those skilled in the art upon review of the following detailed description, claims, and drawings.